[sdiy] CMOS chip questions

[Tom Wiltshire]

On 18 Jul 2009, at 16:43, David G. Dixon wrote:

>> well i meant in a situation where you are trying to replace 74 TTL  
>> with a
>> higher voltage CMOS compatible TTL.
>>
>> i suppose i should just learn to design with actual 5 volt 74LS
>
> I think the most convenient way to go when mixing digital and  
> analog is
> simply to add a 5V supply to the board for digital IC's with  
> 78L05.  Then
> you can use whatever digital IC's are best for a given application,  
> and you
> don't need a stand-alone 5V supply.
>
> Of course, the gurus may beg to differ.  I'd be interested in  
> hearing the
> arguments against this notion.

I'm not claiming "Guru" status just yet either, but that's what I'd  
do. If you've got a lot of logic or digital stuff, then add a +5V to  
your power supply. Otherwise, just stick a regulator on the board  
that needs it. It works fine.
I suppose you could make a case that individual +5V regulators on  
each circuit board (rather than one global +5V supply) might help  
isolate any digital noise (since digital supplies are not shared).  
Still, I've done it both ways in the past and not seen much difference.

For what it's worth, Bruce Duncan uses individual +5V regulators with  
his digital Modcan modules. He might qualify for "Guru" status!
(Care to chip in, Bruce?)

HTH,
Tom

(Just come back from SDIY UK in Cambridge)